An optical film having a hard coat layer (hereafter referred to a hard coat film) is provided on an outermost surface protection of display devices such as a cathode ray tube display device (CRT), plasma display (PDP), electroluminescence display (ELD) and a liquid crystal display device (LCD) for the purpose of improving surface protection, generally. The hard coat film is produced by providing a hard coat layer composed of a photopolymerizable resin, thermosetting resin or UV ray curable resin on a transparent film substrate such as a cellulose acetate based resin (triacetylcellulose in majority) and poly ethylene terephthalate.
The hard coat film usually requires to have certain quantity of thickness of a hard coat layer from a view point of mechanical strength (such as pencil hardness, anti-abrasion property), and recently, film having high pencil hardness (high hardness) is desired. However, there is a problem when thickness is increased to impart high hardness, while pencil hardness is improved, adhesive properties between a hard coat layer and transparent film substrate is not obtained.
Particularly in case of a liquid crystal display which is frequently used outdoors, it is required to also have a property not to deteriorate due to environmental change in addition to a high hardness (resistance to abrasion). However, adhesive properties between a hard coat layer and transparent film substrate are hard to obtain and further surface hardness decreases markedly in the condition supposing outdoor use in particular (light irradiation after a durability test such as a cyclic thermo test).
For the above described problem, an optical multi-layer material is disclosed, which is composed of a hard coat layer provided on a light transmissible resin substrate, wherein the above described hard coat layer is a layer hardened by containing (1) a resin having a weight average molecular weight between 1000 and 100,000, as well as at least two radical polymerizable functional groups, and (2) a resin having a weight average molecular weight between 100 and 1,000, as well as at least one cationic polymerizable functional group, and further, the above described resin (2) is penetrated into the above described light transmissible resin substrate and is hardened (see, Patent Document 1).
According to the above described technology, reflection at interface and interference pattern are prevented, as well as shrink wrinkling of light transmissible resin substrate in inhibited, visibility is improved and further, curing is inhibited, sufficient an anti-abrasion property is maintained, and adhesive properties between a light transmissible resin substrate and a hard coat layer is excellent However, according to the above described technology, adhesive properties after a durability test such as a cyclic thermo test are insufficient and decrease of surface hardness is large.
The liquid crystal display device is usually composed of a liquid crystal cell having transparent electrodes, liquid crystal layer, a color filter and the like between glass plates and two polarizing plates provide on both sides of the liquid crystal cell, and each of polarizing plates is composed of a polarizer (referred to also a polarizing membrane or a polarizing film) being placed between two optical films (a polarizing plate protecting film). Cellulose triacetate film is usually used for the polarizing plate protecting film.
However, it is difficult to obtain sufficient resistance to humidity by the cellulose ester film such as cellulose triacetate film used hitherto. And there is a problem that an optical influence becomes large when the thickness is raised to obtain high resistance to humidity. Further, it is problem that the polarizing plate itself becomes thicker since thinning device is required in recent years.
On the other side, polymethyl methacrylate (PMMA), which is a representative acrylic resin and exhibits excellent transparency and dimensional stability in addition to low moisture absorptive performance, is suitably used in the optical film as material for a low moisture absorptive optical film.
However, format size of a liquid crystal display device is enlarged as described above and usage is expanded to outdoor. According to these circumstances light quantity of the back light is required to increase to sufficiently observe image outdoors, and heat resistance at high temperature or heat resistance for longer time is required due to usage in a severe condition.
However, PMMA film is poor in heat resistance and problems is caused because shape changes due to use at high temperature or longtime.
This is a problem of physical property of film it self as well as a significant problem in a polarizing plate or a display device using the film. That is, a problem that a panel as whole bents is caused when a polarizing plate bents according to deformation of a film in a liquid crystal display device. The problem due to film deformation is also problematic on the back light side, and further, there are problems that design phase difference changes due to deformation, causing fluctuation of view angle and hue change in case it is used at a position of observing surface side.
Further, the acrylic resins film has fragile and brittle property in comparison with cellulose ester film etc. and is difficult to handle, and particularly, is difficult to manufacture an optical film for a large format liquid crystal display device stably.
For the above described problem a method to add polycarbonate (referred to PC) to acrylic resins to improve resistance to humidity and heat is proposed, however it is difficult to use for an optical film since applicable solvents are limited, miscibility between resins is insufficient and easily cause white turbid (see, for example, Patent Document 2).
There are other methods to improve heat resistance of introduce an alicyclic alkyl group as an acrylic resin copolymer component, forming a cyclic structure on a main chain of a molecule by intramolecular cycle forming reaction (see, for example, Patent Document 3 and 4).
However, heat resistance can be improved, but brittleness of a film is insufficient, and it is difficult to manufacture an optical film for a large format liquid crystal display device by these methods. Further, when brittleness is insufficient, an optical film enhance panel deformation, as its result, change of phase difference cannot be inhibited and further, problems of variation of view angle and hue are caused.
As a technology to improve resistance to humidity and heat, resin in which anti-impulsive acryl rubber-methylmethacrylate copolymer or butyl modified acetyl cellulose are combined with acrylic resins is proposed (for example, Patent Document 5).
However, improvement of sufficient brittleness cannot be obtained and handling property is not insufficient for manufacturing an optical film for a large format liquid crystal display device by this method. Further, haze generates due to blended component, a problem of reducing image contrast was found when used outdoors where higher contrast is required.
Further, a technology to blend a resin such as poly vinyl acetate or PMMA with a conventional cellulose ester film to control an optical property (for example, Patent Document 6).
However, according to this method, heat resistance or transparency can be obtained, wettability to aqueous adhesive composed of mainly polyvinylalcohol (referred to PVA) is not good during manufacturing process of an optical film in comparison with conventional cellulose ester, adhesive properties to a polarizer is supposed to reduce, and there is a possibility to cause peeling from a polarizer particularly allowed to be exposed to environment, of high temperature and high moisture for long time.
Further, saponification treatment is required during preparation of a polarizing plate by pasting an optical film to a polarizer in the conventional art disclosed in Patent Document 6, hydrophilic process by saponification is not accelerated in comparison with an optical film composed of cellulose ester resin, there is a problem that adhesive properties between a polarizer and an optical film deteriorate in durability test.
For solving the problem, a measure to improve wettability of adhesive by provide a back coat layer and thereafter saponification is conducted is taken to insure adhesive properties between a polarizer and an optical film.
However, refractive index of a back coat layer reduces when the saponification is conducted whereby difference of refractive index from an optical film is generated and an optical two layer-construction is formed. When the optical film is arranged on outermost surface side of the image device (observation side), visual appearance of a product is not preferable because an optical two layer-construction due to refractive index difference induces interference unevenness. Further, since adhesive properties between a back coat layer and film substrate is not sufficient, and strength is weak, there is a defect to be easily damaged when an optical film provided with a back coat layer is conveyed via rolls.
Under the above described circumstances, problems of the optical film such as low moisture absorptive property, transparency, high heat resistive, brittleness, adhesive property with a polarizer becomes more and more remarkable in accordance with enlargement of usage of recent liquid crystal display device, and improvement is required.
Therefore solution of various problems in relation to an optical film substrate composed of mainly cellulose ester is desired.